The present invention relates generally to a method and an apparatus for optically measuring or determining an amount of electric current/strength of a magnetic field by using Faraday effect. More particularly, the invention is concerned with a method and an apparatus for optically measuring an electric current/magnetic field strength with temperature compensation.
Using Faraday effect, i.e., an magneto-optical effect, to measure the strength of a magnetic field attracts more and more attention of the industrial world. This technique uses a light beam as a mean for the measurement because it is immune to the electromagnetic induction noise. This feature is used for the application of measuring electric current which generates a magnetic field.
Such a magneto-optical technique utilizing the Faraday effect is usually realized by an apparatus having a Faraday element or magneto-optical element, a polarizer and an analyzer, a light source device for generating a light beam, and an opto-electric transducer for converting an optical output from magneto-optical element into an electric signal. The magneto-optical element modulates the incident light beam according to the strength of a magnetic field to which the magneto-optical element is exposed. The modulated light beam is received by the opto-electric transducer.
In the apparatus indicated above, the light beam produced by the light source device is linearly polarized by the polarizer, and the linearly polarized light beam is transmitted through the Faraday element or magneto-optical element in the magnetic field. As a result, the plane of polarization of the linearly light beam is rotated as a function of the strength of magnetic field, due to the Faraday effect. Then the rotated light beam is incident upon the analyzer having a polarizing direction different from that of the polarizer, whereby the angle of rotation is converted into the amount of light which has passed through the analyzer. In other words, a change in the optical output of the Faraday element corresponds to a change in the strength of the magnetic field. As is well known in the art, the optical output of the Faraday element is expressed by a formula which includes the Verdet's constant. According to this formula, the strength of the magnetic field and the amount of electric current which induces the magnetic field, may be determined.
However, the method mentioned above suffers from an inherent problem that the output signal level is influenced by a change in the environments around the apparatus. For instance, the formula used to determine the magnetic field strength including the Verdet's constant of the material of magneto-optical element which has a certain degree of temperature dependence.
Refer to FIG. 1, which is the prior art method of optically measuring a magnetic field or the strength of electric current by utilizing Faraday effect, includes a light source device 10, a polarizer 20, a magneto-optical element(Faraday element) 30, an analyzer 21, and an optical-electric transducer 11. These elements are arranged along an optical path of the light source device. The light source device 10 emits a light beam 15A to polarizer 20. The light beam coming out of the polarizer 20 becomes linearly polarized beam 15B and then passes through a magneto-optical element 30 with length L. Under the effect of a magnetic field H, the linearly polarized light beam 15B when passing through the magneto-optical element 30 is thereby rotated to an angle .THETA. on its plane of polarization. The polarizedly rotated beam 15C is directed through an analyzer 21 which outputs the optical signal P with modulated intensity corresponding to the rotation angle. The optical signal P is then converted by an optical-electric transducer 11 into an electric signal, which stands for the measurement of magneto-optical element. The relationship of angle e and the biased optical signal P may be represented by the following formulas: EQU .THETA.=Ve*H*L (1) EQU P=Po(1+sin(2.THETA.))=Po(1+sin(2*Ve*H*L)) (2)
where
Ve=Verdet's constant PA1 H=strength of magnetic field PA1 L=length of Faraday element
Also, the angle between the polarizer and the analyzer is 45.degree..
From equation (1) and equation (2), it is obvious that the rotation angle .THETA. varies with Verdet's constant, and the optical signal P is also related to Verdet's constant. Because of this Verdet's constant, the prior art is also temperature dependent. In other words, the measurement of the prior art will be influenced by the operating environments. However, this drawback may be compensated by employing the concept of temperature compensation as the one introduced in this invention so that the result of measurement is independent of operating environments.